Heat-exchanger devices of tubular, planar and other configurations with a porous metallic surface layer have been proposed in the art. Such a layer is known to provide a highly efficient thermal interface by virtue of the extended effective surface areas for transferring the heat compared with smooth and conventional finned thermal interfaces. Furthermore, with the porous interface in contact with a boiling liquid, the individual pores or cavities when properly sized and distributed serve to provide highly effective sites for bubble nucleation and thus promote the nucleate bubbling (bubble forming and growing) process. As a result, effective heat transfer coefficients ten times greater than conventional fin-type members or even more can be obtained.
Heretofore, various techniques have been applied in order to produce heat-transfer interfaces of a desired porosity with necessary mechanical properties but have been found to be more or less defective or unsatisfactory especially where they are to be exploited for production on a commercial scale. Especially it has been extremely difficult to produce a large quantity of desired porous heat-transfer members of uniform quality economically. With the spray coating technique in which fused metallic particles are sprayed upon a substrate, it is difficult to obtain a structure which possesses both sufficient and uniform porosity and adhesiveness. Sintering, brazing or soldering metallic particles onto a substrate involves relatively complicated steps which are hard to automate and consequently have rendered the products costly. Furthermore, they may be impractical when curved or elongated tubings or other surfaces of intricate geometries are to receive particles intimately bonded thereto. Where a desired porous structure is to be formed integral with the inside wall of a tubular member, honeycomb or other irregularly shaped surfaces, any of these techniques cannot be effectively utilized.